Mr Walker was sacked from his post in February 2010, allegedly for swearing in meetings.

He has always maintained the real reason was his refusal to bend to pressure from East Midlands Strategic Health Authority (SHA) to prioritise hitting waiting list targets. Mr Walker argued this would have endangered the safety of emergency patients.

He eventually received a £325,000 pay-off from the trust, on the condition he never talked about the dispute.

Yesterday he gave evidence about the ‘gagging clause’ and the build up to his dismissal.

Describing the threats he claimed to have been subjected to, he said: “I wrote to Sir David Nicholson explaining everything that had gone on.”

He said he had “asked for protection as a whistleblower”, told him about “threats to patients’ safety" about "being forced to comply with targets” and said the SHA was “bullying members of staff”.

However, he said: “Yesterday at the Public Account Committee, David Nicholson denied all of that.”

The letter, which Mr Walker presented as evidence to the health committee, concludes: “I assume the Department of Health has a policy on whistle-blowing and would therefore like this letter to be considered in that context and not freely copied to the SHA or the local PCT [primary care trust].”

Threats included that his "career would be in tatters" if he did not agree to leave his post, he told Sir David.

He said: "Bullying and harassment is something I, my chairman and other directors have experienced from the SHA."

Referring to the scandal at Mid Staffordshire NHS Foundation Trust, which at the time was just unfolding, he added: “This is the behaviour that gave this country a mid-Staffordshire.”

On Monday Sir David referred to the letter as an example of how he had “intervened immediately” to address concerns, after Mr Walker had written to him with concerns.

He added: "I have to say he didn’t identify himself as a whistleblower at that moment in time, nor did he raise with me any issues of patient safety.

“Nevertheless, I immediately responded by commissioning an independent inquiry into the circumstances.”

Steve Barclay MP specifically asked Sir David if Mr Walker suggested in the letter he should be regarded as a whistleblower. “No,” he responded.

Sir David today wrote to Margaret Hodge MP, chairman of the PAC, to "correct one specific point of detail".

He wrote: "In response to a question from Stephen Barclay MP about Gary Walker, I said that when he initially wrote to me, I thought that he did not identify himself as a whistleblower.

"I have now had the opportunity to review the correspondence and would like to confirm that when Gary Walker wrote to me in July 2009 he did indeed ask to be considered as a whistleblower."

He continued: "However, I would reassure you that the action I took at the time in response to Gary’s letter was appropriate.

"As I explained yesterday, I immediately commissioned a comprehensive independent investigation into the allegations that had been made.

"The investigation was undertaken by a senior NHS manager (Neil Goodwin) and a senior non- executive (Susan Pyper). A summary of their report was published at the time and their full report has recently been put into the public domain."